So you would pick an arbitrary upper limit based on your own notion of what is sane. Picking such a limit, you would still need to write the same error handling code for limits, but it would happen at maybe 200. And the next programmer who inherits your code looks at the spec and your code and has to guess "why 200"? And it becomes lore. Which is specifically worse than writing to the spec.

The spec doesn't exist. It's undefined. "Must support up to 100" is not a limit, it's a lower bound.